[wasm] Stage --wasm-shared-engine behind --future.

R=titzer@chromium.org
BUG=v8:7424

Change-Id: I49f0f31e19d9242ac6eb26406a0aecd4d60ad869
Reviewed-on: https://chromium-review.googlesource.com/1148396
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54815}

src/flag-definitions.h

@@ -615,6 +615,7 @@ DEFINE_BOOL(wasm_no_stack_checks, false,
 
 DEFINE_BOOL(wasm_shared_engine, false,
             "shares one wasm engine between all isolates within a process")
+DEFINE_IMPLICATION(future, wasm_shared_engine)
 DEFINE_BOOL(wasm_shared_code, false,
             "shares code underlying a wasm module when it is transferred")
 DEFINE_BOOL(wasm_trap_handler, true,